Biogeochemical Cycles - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 151

relative concentrations of reactants and by the influence of ambient ionic strength
on the activities of the reacting species. It is, therefore, expected that solubility,
speciation, and partition behavior of organic contaminants might vary as a result of
salinity-induced changes in pH. Occasionally, extreme pH changes might occur as
a result of input of acidic or alkaline waste into coastal waters. When salinity
increases, organic compounds become less soluble in water and, hence, become
more sorbable, leading to increased sorption on sediment particles. 126 The pH
strongly influences adsorption, since hydrogen and hydroxide ions are adsorbed and
the charges of other ions are influenced by the pH of the water. For typical organic
pollutants, adsorption increases with increasing pH. This is important for coastal
lagoons where the pH is usually higher than 8. Normally, the adsorption reactions
are exothermic, which means that adsorption will increase with decreasing temper-
ature, although small variations in temperature do not tend to alter the adsorption
process to a significant extent.
A schematic fate model for organic chemicals in water and sediment is given
in Figure 4.6.
Ney, 127 Kolset et al., 128 Saleh, 129 Rajar, 130 and Schnoor 131 report evaluation of some
chemical fate and transport models. Bowmer, 132 Day, 133 Miles, 134 Saleh, 129 Inaba
et al., 135 and Jaskulke et al. 136 emphasize the evaluation of pesticide residues in water.

4.2.3.1 Volatilization

Volatilization is the transfer of chemicals from the liquid phase to the atmosphere.
It does not result in the breakdown of a substance, only its movement from the
liquid to gas phase, or vice versa. Volatilization is one of the key processes affecting
the transport and distribution of many organic compounds in the environment. 106
Here are few examples of the importance of this process, especially in coastal
lagoons, are:

• When the oxygen concentration in water is higher than the temperature-
dependent equilibrium concentration between water and the atmosphere,
oxygen is transferred from the water to the air. This condition can exist
in eutrophic lagoons during summer when photosynthesis is dominant.
• When the carbon dioxide concentration in water is higher than the equi-
librium concentration between water and atmosphere, surplus carbon
dioxide will be transferred from the hydrosphere to the atmosphere.
However, the transfer of carbon dioxide from atmosphere to water is, in
general, more normal and important than the opposite process. Carbon
dioxide is produced by the combustion of fossil fuel, and about 60% of
the carbon dioxide produced is dissolved in the sea. However, the transfer
of carbon dioxide from water to air is environmentally significant for
water bodies with low pH. 106 Acidification of the water environment
means that bicarbonate is transformed into carbon dioxide, which then
escapes from the water. This implies that carbon can become a limiting
nutrient in those circumstances and the water environment can turn
oligotrophic.